1. Field of the Invention
The present invention relates to an liquid-crystal display panel drive power supply and to a method for reducing the power consumption of this liquid-crystal display panel drive power supply.
2. Description of the Related Art
In recent years, with the widespread use of liquid-crystal display panels in portable electronic equipment, there has been a demand for lower power consumption in a power supply for liquid-crystal displays and for an improvement in the output impedance of a power supply to accommodate a large liquid crystal panel for display of special characters. FIG. 6 shows a block diagram that includes a liquid-crystal display panel and the peripheral drive circuitry therefore. The display panel M4 is formed by sandwiching a liquid crystal between two glass electrodes that have a multitude of parallel wires such that the electrode lines are mutually perpendicular.
Of the two electrodes, a first electrode, the common (COM) electrodes, are usually taken from the lateral direction of the panel, and the second electrodes, the segment (SEG) or data electrodes, are usually taken from the vertical direction.
The points at which the common electrode intersects with the segment electrode with the liquid crystal therebetween form an equivalent capacitance (hereinafter referred to as a pixel capacitance), and by applying a prescribed potential difference between each of the common and segment electrodes, a potential is applied to corresponding pixel capacitance, resulting in display of that pixel. Therefore, by selecting the potential of the segment electrodes in accordance with display data while scanning (selecting) the common electrodes, it is possible to display data. The selection circuit M2, the common driver M3, and the segment driver M5 are basically formed by analog MOS switches, a prescribed level of power supply circuit M1 being selected in accordance with the scanning and data display timing, so as to apply voltages to the electrodes of the liquid-crystal panel. FIG. 7 shows an example of output waveforms for the case in which the voltages V1 to V5 which are generated by the level power supply circuit =M1 of FIG. 6 and VEE (ground) are output by the common driver M3 and the segment driver M5. The segment driver M5 outputs as a selected level (V1 or ground) or non-selected level (V3 or V4) in accordance with the existence or non-existence of data. Because when the voltages which is applied to a liquid crystal are applied in a DC manner, the deterioration of the liquid crystal is accelerated, in general the selected and non-selected levels are varied with a given period, so that they are applied as AC levels. FIG. 7 is an example in which selected level and non-selected level are changed for each common scan, this being known as the frame reversal mode. For this reason, driving a liquid crystal requires the use of a multilevel power supply. However, with the use of liquid-crystal displays in portable equipment, it is also necessary for the liquid-crystal display power supply to have low power consumption. Because of this need, a circuit such as shown in FIG. 9 was used in the past as a power supply circuit. In FIG. 9, to limit wasteful power consumption other than for driving the liquid-crystal, voltage-dividing resistors R1 through R5 are established with resistance values in the range from several tens of kilohms to several hundreds of kilohms, thereby limiting the current flowing in the idling condition.
However, if the output impedance is high, driving a liquid crystal, which represents a capacitive load, results in waveform distortion, this resulting in a deterioration of display quality. Because of this, the divided voltages are output via amplifiers (B1 through B5), so that there is an improvement in the charging capacity and discharging capacity at the voltage levels required for liquid crystal drive. However, in order to limit the increase of current consumption caused by the use of amplifiers, an external bias is used with each amplifier to limit the bias current, thereby limiting internal current and unnecessary current. FIG. 10(a) shows the charging capacity, while FIG. 10(b) shows the discharging capacity of an amplifier, and in the prior art example of FIG. 9, the amplifiers B1, B2, and B4 have the configuration of FIG. 10 (a), while the amplifiers B3 and B5 of FIG. 9 have the configuration of FIG. 10 (b). The power supply voltages are the maximum potential within the circuit (VLCD) and the minimum potential (GND). FIG. 7(c) is a specific example of segment output waveforms for display and non-display that are repeated. If the time when the common selection level is the maximum drive potential V1 is frame 1 and the time when the common selection level is the minimum potential GND is frame 2, during frame 1 the segment is selected between V4 and GND, while during frame 2 the segment is selected between V1 and V3. If we observe one segment, this segment has n intersections between n commons, meaning that it has n display pixels (capacitances) with respect to common. Because only a single common outputs a selection level during a given frame, only one terminal that is different from the above-noted pixel capacitance segment is shorted to common, with the remaining nxe2x88x921 being shorted to the non-selected level. FIG. 8(a) illustrates the condition of the current flow in the power supply that outputs the voltage levels V1 and V3 when the common and segment drivers operate, in the power supply that is shown in FIG. 9, when the frame 2 operation of FIG. 7 (c) is done. Here, if the capacitances CL1 and CL2 per pixel are Cp, CL1=(nxe2x88x921)xc3x97Cp and CL2=Cp. As the panel becomes larger (that is, as n increases), the load capacitance increases, this leading to an increase in the equivalent capacitance at each level, making it necessary to lower the output impedance sufficient so that it is possible to provide sufficient drive for the capacitive load. However, with the reduction of power consumption equipment using liquid-crystal displays in recent years, even the bias current becomes significant.
For example, in the case in which the resistors R1 through R5 are 500kxcexa9, for VI1=10 V, the idling current flowing in the resistances can be limited to 10 V/(500kxcexa9xc3x975)=4xcexcA. However, in the differential and output stages of the amplifiers of FIG. 10(a) and FIG. 10(b), in the bias current is 1 xcexcA, the overall amplifier bias current in the power supply circuit is (1+1)xc3x975=10 xcexcA. This current flows even when a load is not being driven, and is thus wasteful, and this has represented a technological problem with the move to lower power consumption in drive power supplies in recent years.
In this type of circuit, because charging and discharging by the amplifier of the liquid crystal load is performed between the internal circuit maximum potential (VLCD) and minimum potential (GND), regardless of the voltage level to which charging and discharging is done, this is basically merely discharging via the MOS output stage of the amplifier to the maximum potential (VLCD) or the minimum potential (GND) and this circuit does not make re-use of load current. However, according to an example of prior art as disclosed in Japanese Unexamined Patent Publication (KOKAI) No.5-257121, as shown in FIG. 11, there is a circuit that takes each of the potentials that are divided by resistors as the power supply voltages of the amplifiers. In this circuit, the current from an amplifiers flows into divided resistances, this resulting in a deterioration of display quality according to level change. Because the amplifier power supply has an impedance of 5 kxcexa9or greater (in the prior art example, R1 is 5 kxcexa9to 15 kxcexa9), not only does the output impedance (sum of the power supply impedance and on resistance of the output buffer) rise to greater than the divider resistances, but also the high power supply impedance results in unstable amplifier operation, due to noise, for example. If the output impedance of the amplifier is limited, there is a reduction in the above-noted divider resistances, so that the current flowing therein rises, the result being the problem of an increase in current consumption greater than the amplifier.
Accordingly, it is an object of the present invention to improve on the above-noted drawbacks in the prior art by providing a novel liquid-crystal drive power supply circuit which limits the current consumption more than in a liquid crystal drive power supply of the past, while making re-use of the charge that is charged and discharged when a load is driven so as to limit the current consumption during operation, the output level of the amplifier not being caused to vary and the output impedance being lowered so as to improve the quality of the display. Another object of the present invention is to provide a method of reducing the current consumption in the above-noted liquid-crystal drive power supply circuit.
In order to achieve the above-noted object, the present invention adopts the following basic technical constitution.
Specifically, the first aspect of a liquid-crystal display panel drive power supply circuit according to the present invention is a liquid-crystal display panel drive power supply circuit having a first power supply of a high potential, a second power supply of a potential that is lower than the potential of the first power supply, a plurality of voltage-dividing resistors provided in series between the above-noted first power supply and second power supply, and a plurality of voltage-follower configured amplifiers for introducing a plurality of differing voltages from the connection points between the above-noted resistors to a liquid-crystal display panel, wherein a capacitor is connected between an output terminal of each of the above-noted amplifiers and the second power supply.
In the second aspect of the present invention, the output voltage of an amplifier that outputs an output voltage to an output terminal that is higher than the output voltage of the amplifier is taken as the first power supply means, and the output voltage of an amplifier that outputs an output voltage to an output terminal that is lower than the output voltage of the amplifier is taken as the second power supply means.
In the third aspect of the present invention, the output voltage of an amplifier that outputs an output voltage to an output terminal that is higher than the output voltage of the amplifier is taken as the first power supply means and the output voltage of an amplifier that outputs a voltage to an output terminal that is the lowest among the amplifiers that output voltages that are higher than the output voltage of the amplifier is taken as the first power supply means, while the output voltage of an amplifier that outputs an output voltage to an output terminal that is lower than the output voltage of the amplifier is taken as the second power supply means and the output voltage of an amplifier that outputs a voltage to an output terminal that is the highest among the amplifiers that output voltages that are lower than the output voltage of the amplifier is taken as the second power supply means.
In the fourth aspect of the present invention, the output voltage of an amplifier that outputs an output voltage to an output terminal that is higher than the output voltage of the amplifier is taken as the first power supply means and the output voltage of an amplifier that outputs a voltage to an output terminal that is not the lowest among the amplifiers that output voltages that are higher than the output voltage of the amplifier is taken as the first power supply means, while the output voltage of an amplifier that outputs an output voltage to an output terminal that is lower than the output voltage of the amplifier is taken as the second power supply means and the output voltage of an amplifier that outputs a voltage to an output terminal that is not the highest among the amplifiers that output voltages that are lower than the output voltage of the amplifier is taken as the second power supply means.
In the fifth aspect of the present invention, the above-noted amplifier is configured by MOS transistors, which are formed on a substrate which is separated by a dielectric.
In the sixth aspect of the present invention, the above-noted amplifier is configured by MOS transistors, which are formed on an SOI substrate.
An aspect of a method of reducing the current consumption in a liquid-crystal display panel drive power supply is a method for reducing the current consumption in a liquid-crystal display panel drive power supply circuit having a first power supply of a high potential, a second power supply of a potential that is lower than the potential of the first power supply, a plurality of voltage-dividing resistors provided in series between the above-noted first power supply and second power supply, and a plurality of voltage-follower configured amplifiers for introducing a plurality of differing voltages from the connection points between the above-noted resistors to a liquid-crystal display panel, wherein a capacitor is connected between an output terminal of the above-noted amplifier and the second power supply, and a charge that is temporarily stored in this capacitor is re-used as the power supply of another amplifier of these amplifiers, thereby reducing the power consumption.
Embodiments of a liquid-crystal display panel drive power supply according to the present invention can be described with reference to accompanying drawings.
Referring to FIG. 1, in an embodiment of the present invention, in a multi voltage level output power supply circuit for driving a liquid crystal, this being formed by amplifiers (buffers) having an output impedance sufficient to drive a liquid crystal by inputting voltages that are divided by the resistive voltage divider formed by R1 through R5, which divides the voltage between the maximum potential (VI1) and the minimum potential (GND) for operating the liquid crystal, capacitors (C1 through C5) are inserted between the output of each amplifier and an internal circuit potential (GND or VLCD) so as to stabilize the level, and reduce the impedance. The output of an amplifier that outputs a voltage that is higher than this stabilized amplifier output voltage (hereinafter referred to as the high-potential level) is taken as the upper power supply, and the output of an amplifier that outputs a voltage that is lower than the above-noted output (hereinafter referred to as the lower-potential level) is taken as the lower power supply.
Next, the operation of the above-noted power supply circuit will be described, with reference to FIG. 1.
In a circuit of the prior art (FIG. 9), regardless of the output voltage level of the amplifier, a bias current flows within the circuit, from the maximum potential (VLCD) to the minimum potential (GND). The load drive by the output stage is merely one of discharging a charge stored in the load to the minimum potential (GND) or charging the load to the maximum potential (VLCD), with each amplifier consuming current independently. In the present invention, however, because the amplifier power supply is taken as higher than and lower than the output of a given amplifier, the bias current in the highest-order amplifier A1, which has the maximum potential (VLCD) and V2 potential as power supply voltages, flows into the V2 voltage level and is temporarily stored in capacitor C2. In the intermediate potential amplifier A3, because the power supply voltages are V2 and V4, the current that flows into the above-noted V2 voltage level is again stored in the V4 level capacitor C4. Because V4 is the power supply of the minimum-potential amplifier A5, this charge can be used again for the bias current of the minimum-potential amplifier A5. Simultaneously with this, the amplifier A4 can make re-use of the bias current consumed at A2.
In addition to the bias currents, in contrast to the prior art example of FIG. 9, in which the currents (charges) that are consumed in each of the amplifiers in driving the loads are not derived by charging and discharging of the loads to maximum and minimum potentials, each level charge is used, enabling re-use as described with regard to the bias current. By means of charge distribution between the various level capacitors and load capacitances, charges are reclaimed by each level capacitor, enabling their re-use as amplifier currents.